Haversine

//Calculates distance between two sets of Decimal Degree coordinates using the Haversine formula
//Accepts two sets of longitudes and latitudes in decimal degree format and calculates distance between
//in meters.
signed long get_distance(signed long lat1, signed long lat2, signed long lng1, signed long lng2){
    signed long radius = 6371000;   //Approximate radius of the Earth in meters
    double latDif, lngDif, radLat1, radLat2, a, c, d;

//It should be able to handle negatives, but in case it can't...
//  if(lat1 < 0)
//      lat1 *= -1;
//
//  if(lat2 < 0)
//      lat2 *= -1;
//
//  if(lng1 < 0)
//      lng1 *= -1;
//
//  if(lng2 < 0)
//      lng2 *= -1;

//Dividing breaks everything
//  lat1 /= 10000000;
//  lat2 /= 10000000;
//  lng1 /= 10000000;
//  lng2 /= 10000000;

    //Convert from degrees to radians
    radLat1 = lat1 * M_PI / 180;
    radLat2 = lat2 * M_PI / 180;
    latDif = (lat2 - lat1) * M_PI / 180;
    lngDif = (lng2 - lng1) * M_PI / 180;

    //Apply the Haversine formula
    a = sin(latDif/2) * sin(latDif/2) + cos(radLat1) * cos(radLat2) * sin(lngDif/2) * sin(lngDif/2);
    c = 2 * atan2(sqrt(a), sqrt(1-a));
    d = radius * c;

    return (long)d;
}